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:

After:

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