Add LabPlot project file icon

Summary:
Rather than simply displaying the gzip icon, Breeze should include a nice icon for LabPlot files. Here's a screenshot:

It uses the colors from the Breeze palette, Icon Grey, Icon Blue, Icon Red, and for the axes a color that I copied from the Breeze LabPlot icon.

Test Plan:

• Make sure the icon is easy enough to understand

Reviewers: LabPlot, VDG, Breeze, ngraham

Reviewed By: VDG, ngraham

Subscribers: abetts, asemke, ngraham, kde-frameworks-devel

Tags: Frameworks, Breeze

Differential Revision: https://phabricator.kde.org/D13643