Add build script for AppImage about heimer

If you need any help just ask @probonopd ;-)

[probonopd]

I have made a test AppImage using https://github.com/probonopd/Heimer/blob/patch-1/.travis.yml.
It is for download at https://github.com/probonopd/Heimer/releases.
Please test it; if it is OK I will send a pull request to this repository. Thanks!

[juzzlin]

Seems to work except for the launcher icon. There's data/icons/heimer.png that's used for Debian packaging. Is it possible to make that work with AppImage also?

[probonopd]

Please use QApplication::setWindowIcon() in your code.

[juzzlin]

It's the location of the file in the source tree.

The current configuration installs it to ${CMAKE_INSTALL_PREFIX}/share/icons/hicolor/64x64/apps as you can see in InstallLinux.cmake.

The desktop file already has Icon=heimer and this works for the .deb.

[juzzlin]

Actually QMainWindow::setWindowIcon() was already used, but the icon resource file name was not correct. Thanks for noticing this! It seems that Qt doesn't give any warnings about this..

[probonopd]

Thank you very much. Looks like this can be closed now, as an AppImage is available on GitHub Releases. Thank you very much!

Reference:
AppImage/appimage.github.io#771