[KDev4] Improving mainwindow UI

[dukju ahn]

1.
Current KDev4 mainwindow doesn't allow to hide entire docking
window easily -- we should close every 4 tab to remove bottom
docwidget. OTW, in Kdev3.4, we could lower dock window group
by clicking its tab only once.
This is necessary to acquire wide editor window.
But it seems that QMainwindow doesn't support that.

2.
Current default mainwindow corner setting is hardcoded.
Left corners are occupied by left docwidget, right corners are
occupied by right docwidget. Hence bottom docwidget becomes
smaller.
I think there should be a setting page regarding corner occupation.
Or even better, we can invoke context menu and specify corner
occupation dynamically.

Are there anyone who object these idea? What is the current
plan regarding the UI?