Fix include guards and copyright statements

[messmerd]

This PR corrects some minor issues with header files under include/

• Added namespace prefixes (LMMS_ or LMMS_GUI_) to include guards of header files in the include/ directory. This fixes a name conflict between the include guards of include/Engine.h and plugins/ZynAddSubFx/zynaddsubfx/src/Nio/Engine.h and possibly others. It will also prevent any future conflicts.
• The include guards of Clip.h and ClipView.h were still using their old "track content object" names from before the reorganization. This has been corrected. (#5592)
• Added missing copyright statement to MidiCCRackView.h.
• Added missing copyright statements to CustomTextKnob.h and CustomTextKnob.cpp according to author's comment in #5321.
• Makes the number of newlines between copyright statement and include guard, between include guard and includes, and between closing include guard and EOF consistent for all headers

[Veratil]

LGTM

My only note is a few of the #endif's lost their // GUARD_H comment. We don't really need them since they're EOF, but it doesn't hurt to have I think?

[messmerd]

My only note is a few of the #endif's lost their // GUARD_H comment. We don't really need them since they're EOF, but it doesn't hurt to have I think?

There were only a few instances of those, so I just removed them for consistency with the rest of the headers.

[messmerd]

Actually, I'll go ahead and add them to all the headers. It would be better to have them, and now's the best time to do it.

[PhysSong]

Generally looks good.
I'm not sure if it's necessary, but I think it's a good chance to rename some include guards, ex. LMMS_AUDIO_DEVICE_H to LMMS_AUDIODEVICE_H. Historically, some header files used names in snake_case, but they were renamed to PascalCase.

[Veratil]

Since @PhysSong has soft approval with comment, and @DomClark has approval. I'm merging this now.