Fixes bug where clicking on the Activity Indicator doesn't play a note

[IanCaio]

After merging of #5581 a bug was unveiled, where an ambiguity on the arguments types of calls to the MidiEvent constructor would result in ADL picking the wrong option.

Discussing with others in Discord about approaches to fix the issue, @DomClark suggested using an enum class instead of a boolean on the last argument, replacing it from ignoreOnExport to source (representing the source of a MidiEvent). One of the arguments type from the second constructor was also changed from int to std::size_t to better represent it.

Those changes alone fixes the Activity Indicator. It's possible though that adding static_casts to the constructor calls could still be needed. For now I'm submitting the changes for further review and discussion.

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Linux

• Linux (AppImage): lmms-1.3.0-alpha.1.40%2Bga52ab90-linux-x86_64.AppImage (build link)

Windows

• Windows 32-bit: lmms-1.3.0-alpha.1.40%2Bga52ab9070-mingw-win32.exe (build link)
• Windows 64-bit: lmms-1.3.0-alpha.1.40%2Bga52ab9070-mingw-win64.exe (build link)
• Windows 32-bit: lmms-1.3.0-alpha-msvc2017-win32.exe (build link)
• Windows 64-bit: lmms-1.3.0-alpha-msvc2017-win64.exe (build link)

macOS

• macOS : lmms-1.3.0-alpha.1.40%2Bga52ab9070-mac10.14.dmg (build link)

🤖

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[DomClark]

Looks good, and fixes the bug. There are a couple more things that could be done, but I'm not sure if it's worth doing them here:

• m_data.m_sysExDataLen should be changed to a std::size_t, to match dataLen which is stored in it. However, it seems to be unused, so it doesn't matter too much right now.
• Should all MidiEvents be constructed with the correct source? Currently everything is marked as external, except controller changes. Again, this is fine for now, since most messages go to processOutEvent where the source doesn't matter, but it might be worth correcting at some point. (I would probably make the default source Internal, and specify External in the MIDI backends, to keep the core code cleaner.)