Add copy action for HTML preview #133

Show a custom context menu for the HTML preview that contains a copy
action.
Paint disabled icons in disabled window text color
Merge branch 'feature/copy-rich-text-preview-#133' into develop
Construction not known to VC

(cherry picked from commit f6f661c0ce746ca2d6e616c8612ecd5b5f18cad8)
Fix build for compilers supporting Q_DECL_OVERRIDE

The method QIconEnginePlugin::keys() existed only in Qt 4.x. So for Qt 5,
the build with compilers supporting Q_DECL_OVERRIDE resulted in a build
error.
Use newest version of discount
Remove PRE_TARGETDEPS for shared libraries

Remove PRE_TARGETDEPS from app.pro and app-static.pro for the shared
libraries discount, hoedown and hunspell.
travis: Fix error messages regarding not finding translations
Update to Qt 5.2 on travis ci
Merge branch 'feature/upgrade-qt5.2' into develop
WIP: MarkdownConverter using reveal.js

A MarkdownConverter implementation using reveal.js to generate
a HTML presentation.
Template for presentations and splitting of slides

Implement the splitting of the markdown text into multiple slides and also
use a separate HTML template file for the presentations.
Use data-separator attribute to split slides

Use the data-separator attribute feature of reveal.js to split the
markdown document into multiple slides.
Improve handling of ENABLE_HOEDOWN in option dialog

Don't not remove the general tab in the option dialog anymore. Instead
just remove the hoedown item from the combo box.
Use CDN for stylesheets
Add support for MathJax and Highlight.js plugins

Add support for MathJax and Highlight.js through Reveal.js plugins instead
of adding the scripts to the HTML header.
Add abstract Template class and implementations

Move the handling and rendering of the template to its own class. The
implementations are not yet complete.
Use https to fix problem with proxy preventing access here
Complete Template implementations and add support for "Export to HTML"
Add more tests to DiscountMarkdownConverterTest