Use search field for search.

Search options are presented in a context menu on the search field, as well as
links in search header. A 'header' is still displayed while in search mode, to
clearly indicate the mode.

This provides a slightly simpler (no extra click), more common UI, while keeping
the all the features of the earlier search mode.
Merge remote-tracking branch 'Typz/searchfield'

Conflicts:
	src/CMakeLists.txt
	src/gui/DatabaseWidget.cpp
	src/gui/DatabaseWidget.h
	src/gui/MainWindow.ui
Fix deps for qt5