Improve the display of missing projects in the Project Manager
Improve the display of missing projects in the Project Manager

- Use a "broken file" icon instead of a "folder" icon for missing
  projects
- Fade out the project icon for missing projects
- Use transparent modulate for grayed out projects as it results in
  better-looking text
- Disable the "Edit", "Run" and "Rename" buttons if at least
  one missing project is selected
- Make double-clicking or pressing Enter do nothing when a missing
  project is selected
- Change the "Open in FileSystem" tooltip accordingly for missing
  projects
- Don't connect the "Open in FileSystem" pressed signal to make the
  button ineffective for missing projects
- Name missing projects as "Missing Project" instead of
  "Unnamed Project"

Valentactive/godot,Zylann/godot,ex/godot,Zylann/godot,ZuBsPaCe/godot,BastiaanOlij/godot,ex/godot,pkowal1982/godot,vkbsb/godot,firefly2442/godot,guilhermefelipecgs/godot,ZuBsPaCe/godot,godotengine/godot,ex/godot,ZuBsPaCe/godot,Paulloz/godot,ex/godot,Valentactive/godot,DmitriySalnikov/godot,Valentactive/godot,firefly2442/godot,sanikoyes/godot,honix/godot,BastiaanOlij/godot,firefly2442/godot,Valentactive/godot,BastiaanOlij/godot,pkowal1982/godot,Faless/godot,vkbsb/godot,Zylann/godot,Faless/godot,pkowal1982/godot,josempans/godot,sanikoyes/godot,akien-mga/godot,godotengine/godot,MarianoGnu/godot,firefly2442/godot,pkowal1982/godot,BastiaanOlij/godot,vnen/godot,Shockblast/godot,godotengine/godot,Paulloz/godot,akien-mga/godot,vnen/godot,guilhermefelipecgs/godot,Paulloz/godot,Shockblast/godot,godotengine/godot,Zylann/godot,akien-mga/godot,sanikoyes/godot,Faless/godot,guilhermefelipecgs/godot,Shockblast/godot,BastiaanOlij/godot,Shockblast/godot,godotengine/godot,ZuBsPaCe/godot,firefly2442/godot,Valentactive/godot,Faless/godot,Faless/godot,vnen/godot,honix/godot,ZuBsPaCe/godot,josempans/godot,Faless/godot,vkbsb/godot,ex/godot,DmitriySalnikov/godot,honix/godot,vkbsb/godot,ZuBsPaCe/godot,honix/godot,guilhermefelipecgs/godot,Paulloz/godot,vnen/godot,DmitriySalnikov/godot,ex/godot,Paulloz/godot,Paulloz/godot,MarianoGnu/godot,Faless/godot,MarianoGnu/godot,ZuBsPaCe/godot,Shockblast/godot,godotengine/godot,Shockblast/godot,pkowal1982/godot,guilhermefelipecgs/godot,guilhermefelipecgs/godot,Faless/godot,ex/godot,Valentactive/godot,akien-mga/godot,sanikoyes/godot,pkowal1982/godot,firefly2442/godot,BastiaanOlij/godot,akien-mga/godot,Zylann/godot,guilhermefelipecgs/godot,Shockblast/godot,josempans/godot,vnen/godot,DmitriySalnikov/godot,sanikoyes/godot,DmitriySalnikov/godot,honix/godot,DmitriySalnikov/godot,josempans/godot,vkbsb/godot,sanikoyes/godot,Zylann/godot,Paulloz/godot,sanikoyes/godot,guilhermefelipecgs/godot,MarianoGnu/godot,godotengine/godot,DmitriySalnikov/godot,MarianoGnu/godot,vnen/godot,BastiaanOlij/godot,BastiaanOlij/godot,vkbsb/godot,pkowal1982/godot,firefly2442/godot,Shockblast/godot,akien-mga/godot,josempans/godot,vkbsb/godot,ex/godot,ZuBsPaCe/godot,josempans/godot,godotengine/godot,vkbsb/godot,Zylann/godot,vnen/godot,akien-mga/godot,josempans/godot,akien-mga/godot,pkowal1982/godot,MarianoGnu/godot,MarianoGnu/godot,josempans/godot,vnen/godot,Valentactive/godot,firefly2442/godot,sanikoyes/godot,Valentactive/godot,MarianoGnu/godot,Zylann/godot,honix/godot