mainmenu: Reduce delay of popup when using shortcut
mainmenu: Reduce delay of popup when using shortcut

The delay is necessary to ensure that the menu is focused when it pops up.
See <https://github.com/lxde/lxqt-panel/pull/131> and
<https://github.com/lxde/lxqt-panel/pull/312> for more discussion.

Ilya87/lxqt-panel,stefonarch/lxqt-panel,stefonarch/lxqt-panel,pmattern/lxqt-panel,ThomasVie/lxqt-panel-de,pmattern/lxqt-panel,ThomasVie/lxqt-panel-de,stefonarch/lxqt-panel,cferr/lxqt-panel,cferr/lxqt-panel,lxde/lxqt-panel,lxde/lxqt-panel,Ilya87/lxqt-panel