LoadCoreDialog: only try reading from existing corefile
LoadCoreDialog: only try reading from existing corefile

- currently gdb is called on every change of the core file, i.e. on
  every keypress when typing a path. Changed to only do that when the
  file exists

Change-Id: I27f1c21b2561420d72f273244dc6a1bb5fb8bfe7
Reviewed-by: hjk <f81b91af79013637d070c18d38a75b90eaae8ba3@nokiamail.com>

maui-packages/qt-creator,xianian/qt-creator,martyone/sailfish-qtcreator,farseerri/git_code,Distrotech/qtcreator,danimo/qt-creator,AltarBeastiful/qt-creator,maui-packages/qt-creator,martyone/sailfish-qtcreator,maui-packages/qt-creator,Distrotech/qtcreator,martyone/sailfish-qtcreator,danimo/qt-creator,kuba1/qtcreator,kuba1/qtcreator,danimo/qt-creator,Distrotech/qtcreator,kuba1/qtcreator,xianian/qt-creator,AltarBeastiful/qt-creator,xianian/qt-creator,Distrotech/qtcreator,danimo/qt-creator,kuba1/qtcreator,maui-packages/qt-creator,AltarBeastiful/qt-creator,farseerri/git_code,amyvmiwei/qt-creator,amyvmiwei/qt-creator,xianian/qt-creator,AltarBeastiful/qt-creator,maui-packages/qt-creator,danimo/qt-creator,danimo/qt-creator,martyone/sailfish-qtcreator,amyvmiwei/qt-creator,kuba1/qtcreator,martyone/sailfish-qtcreator,Distrotech/qtcreator,kuba1/qtcreator,xianian/qt-creator,danimo/qt-creator,danimo/qt-creator,danimo/qt-creator,farseerri/git_code,kuba1/qtcreator,maui-packages/qt-creator,amyvmiwei/qt-creator,farseerri/git_code,kuba1/qtcreator,AltarBeastiful/qt-creator,martyone/sailfish-qtcreator,farseerri/git_code,amyvmiwei/qt-creator,AltarBeastiful/qt-creator,kuba1/qtcreator,martyone/sailfish-qtcreator,Distrotech/qtcreator,AltarBeastiful/qt-creator,amyvmiwei/qt-creator,farseerri/git_code,amyvmiwei/qt-creator,martyone/sailfish-qtcreator,xianian/qt-creator,farseerri/git_code,amyvmiwei/qt-creator,martyone/sailfish-qtcreator,xianian/qt-creator,AltarBeastiful/qt-creator,xianian/qt-creator,farseerri/git_code,xianian/qt-creator,maui-packages/qt-creator,Distrotech/qtcreator