Code Analysis: Show list of file extensions on which the tool runs on

Summary:
Reason: a long time I wondered how exactly this Code Analysis works and wasn't even aware of that for "cppcheck" .h files are _not_ scanned.

Plus:
- Rearrange to GUI to make the label better fit (at least for my eye): move selector and start
button from the lower right to the upper left.

Before:

{F6451333}

After:

{F6463943}

Ideas for later:
- Make it possible to run a tool for individual files
- Another analyser which derives from the cppcheck class could be created: it specifically sets the language to C++ (in the original class the caption should be renamed to "cppcheck (C/C++) and also includes .h and .hpp files
- Show the number of files the analyser was run on

Reviewers: #kate, cullmann

Reviewed By: #kate, cullmann

Subscribers: cullmann, dhaumann, ngraham, kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D17318