mainmenu: Avoid menu navigation while filtering

[palinek]

Closes #1958

[palinek]

Anyone able to test/review this?

[stefonarch]

I tried to replicate the issue with no success, it has a smell of openbox (which I used for testing).
For the rest the change works fine.

[Chiitoo]

I can re-produce the issue to some extent, using KWin.

If I hover the menu quick-like after typing, I do get the dialogs stay open, but I can also start applications by clicking them there, so not sure this is what the issue is about.

I can still re-produce it with this change as well, but yeah, not sure it is what they are seeing...

[stefonarch]

If I hover the menu quick-like after typing, I do get the dialogs stay open, but I can also start applications by clicking them there, so not sure this is what the issue is about.

I managed to that too, with openbox, but the "phantom" closed by itself fast and I could click too. The issue was different IMO.