Improve virtual keyboard support on top of applications

Currently Kirigami does have basic support for reserving space at the bottom of the application window for keyboards.

Tasks:

Add option to disable the bottom keyboard margin, and let the application handle it. The margin can be very laggy for apps like Angelfish (resizing the webview), and not necessary if the focus is on the search drawer
Opening the keyboard on textfields on the lower edge of the screen causes the keyboard to cover the textfield. While you can scroll to show the textfield, Kirigami should do this automatically
The bottom margin doesn't currently apply to drawers, so the vkbd still covers them. The height should be adjusted so that the textfield is shown right above the keyboard
Elements that are set as the footer of the application window are not addressed properly with the vkbd bottom margin