Inline search and highlight feature for all dropdown lists

[Fish013]

This is an improvement over the original pull request which changes the search from popup to inline style.
It further contains minor refactoring and documentation additions, aswell as fixes to some odd or jarring behavior while filtering.

[NickRyder]

Could you add in a bunch of pictures illustrating the feature set?

[Fish013]

Inline Dropdown Search

Showcase

General behaviour

• Accessible by typing when the dropdown is opened
• The currently selected item text is getting replaced temporarily with the search term
• Filtering out items that do not match the search term
• Highlighting the places where matches in items are found with a thin yellow underline
• Dropdown is resized according to the current number of visible items

Search syntax

• Search term words separated by a space must all match the entry in given order to be shown

Ordered match

Ordered mismatch

• Search does case-insensitive matching and correctly handles special chars

Case-insensitive matching with special chars

Miscellaneous

• Search term is reset when dropdown is closed or ESC is pressed
• Backspace allows you to reword the search term
• When scrolled and the currently selected item is filtered out, the first currently visible item is selected
• Minimum dropdown size is 1 item. If nothing matches "<No match>" is shown in this slot.
• Maximum dropdown size has been reduced from 30 to 20 (this is a buff) to make resizing while filtering feel better