geanyprj: sidebar easy search feature

This branch insert two features in GeanyPrj plugin, both focused on sidebar listing project files.

Simpler keyboard-based item selection in file listing
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When sidebar is focused, user can type extracts of a searched name to easily select first matching file. User can then use up and down arrow to select other matches and 'Enter' key to open it.

As opposite to previous treeview-default behavior, matches are searched anywhere in the filename and not only as a kind of "start-with" test. User can also type two words in order to highlight files containings both substrings in a case insensitive way.


Sidebar file listing filtering
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An optional input box is added on top of file listing to hide non-matching items. User can then press down arrow key to jump in the filtered file listing and search/open one of them.

Configurability
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Few new configuration keys has been added in GeanyPrj plugin configuration pane. Configurable items are:
* search algorithm (Start-with ; Contains ; Contains-all ; Contains insensitive ; Contains-all insensitive)
* filter algorithm (Start-with ; Contains ; Contains-all ; Contains insensitive ; Contains-all insensitive)
* filter availiability

Limitations
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Insensitive algorithm are not available on systems not offering strcasestr() function. In such systems, "Contains-all" is the default algorithm instead of "Contains-all insensitive".