Fix #306512: Improvements to the splash screen

[Spire42]

Resolves: #306512

Rewrote the splash-screen code so that it renders everything on the fly using vector graphics instead of using pre-rendered images.

Also designed and implemented a darker version of the splash screen that is automatically used for unstable/development builds.

• I signed CLA
• I made sure the code in the PR follows the coding rules
• I made sure the code compiles on my machine
• I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"
• I made sure the commit message title starts with "fix #424242:" if there is a related issue
• I created the test (mtest, vtest, script test) to verify the changes I made

[mattmcclinch]

If you rename Maintenance.svg to maintenance.svg, this might actually build on filesystems where case matters.

[Spire42]

Thanks; fixed.

[Jojo-Schmitz]

Now Travis CI got lost once again

[ecstrema]

Would someone mind posting a screenshot? :)

[mattmcclinch]

Stable version:

Unstable version: