Start of find & replace function

Add widget beneath text editor that shows the
find & replace UI.
Add 16x16 icon for close buttons
Improve find/replace function

It's now possible to find forward and backward in the
markdown text.
Add several new icons to icon engine
Add icon to find/replace action
Implement replace & replace all functions

The replace and replace all buttons in the find/replace widget are working
now.
Merge branch 'feature/find-replace' into develop

Closes issue #21