[patch] Search GUI improvements

[bilbo]

This patch improves search GUI in several ways:

Search box is put on top of the dialog and set to be as wide as possible. This makes it much easier to edit long expressions. The search box was way too narrow before.

Historyaware search box is fixed to respect the max history size in options instead of using hardcoded value of 15

All autocomplete boxes (not only the search box, actually, as other boxes share the code too) are improved by increasing the maximum popup size (while still fitting on the screen), so it will contain more items without need to scroll

[bilbo]

Patch to improve search GUI

[bastiK]

In [4018/josm]:

applied #6187 - Search GUI improvements (patch by bilbo)