Add insert/remove bar buttons in Song editor

[allejok96]

Added two buttons for the shortcuts insert and remove bar #5602

This is my first ever pull request and piece of C++ code, so forgive any errors.

[LmmsBot]

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Linux

• Linux (AppImage): lmms-1.2.3-720%2Bg86c1ba4-linux-x86_64.AppImage (build link)

Windows

• Windows 32-bit: lmms-1.2.3-720%2Bg86c1ba43f-mingw-win32.exe (build link)
• Windows 64-bit: lmms-1.2.3-720%2Bg86c1ba43f-mingw-win64.exe (build link)
• Windows 32-bit: lmms-1.2.2-msvc2017-win32.exe (build link)
• Windows 64-bit: lmms-1.2.2-msvc2017-win64.exe (build link)

macOS

• macOS : lmms-1.2.3-720%2Bg86c1ba43f-mac10.13.dmg (build link)

🤖

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[JohannesLorenz]

@allejok96 I get an error while starting LMMS:

Error loading icon pixmap "insert_bar": File not found
Error loading icon pixmap "remove_bar": File not found

Indeed, I see not pixmaps on the new buttons. Did you forget to add them to your commit?

Edit: I see the pixmaps are in default theme, which I have selected ✔️ However, they are not installed yet.

[allejok96]

@JohannesLorenz Will the error fix itself or do I need to change anything?
And does classic theme need the icons too? Ah, it grabs them from default if they are missing...

[JohannesLorenz]

Will the error fix itself

Indeed! I had to do a make clean and a further make install to get it run. Seems like a bug of the build system.

Now going on with the review...

[JohannesLorenz]

LGTM