Show Configure KDevelop... dialog faster

There is an annoying 1.5-second delay before KDevelop settings dialog
appears. This commit reduces the delay by 600 ms.

I have measured the time spent in UiController::showSettingsDialog() -
from the beginning of the function until the last blocking statement
cfgDlg.exec() - with QElapsedTimer. This time is about 300 ms longer
when the Configure KDevelop... dialog is first shown after KDevelop
starts. The following table compares these average times in milliseconds
before and at this commit:
                    before  at
first shown         1530    910
non-first shown     1230    620