Introduce ArgumentHintWidget

This change replaces the old KateArgumentHintTree with ArgumentHintWidget which is a simpler way to show the same data. The problems with old widget were:

• It didn't work well with methods that have many overloads and long or verbose signatures. There was too much visual noise
• It didn't work well with our new approach of not using top level window for completion to avoid wayland bugs as the widget would get cut off at the window edge
• It was too complicated code wise

The new widget is a simple plain text editor that shows the method signature and allows switching between different available signatures using Alt+Up/Down.