Add CMT Submodule

[tresf]

CMT stands for "The Computer Music Toolkit" LADSPA Plugin and we've bundled it with LMMS for quite a while. This PR moves the source code to a dedicated submodule.

• New repo created https://github.com/lmms/cmt, which is a copy of cmt from ladspa.org, but with version history as well as our custom patches added
• Switches plugins/LadspaEffect/cmt` to use a submodule
• Bumps LMMS from CMT 1.16 to 1.17
• Fixes a bug with 1.17 where it won't compile with Clang (and possibly others)

Related #3963

[LmmsBot]

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Linux

• Linux (AppImage): lmms-1.2.3-750%2Bgb1a9050-linux-x86_64.AppImage (build link)

Windows

• Windows 32-bit: lmms-1.2.3-750%2Bgb1a9050a3-mingw-win32.exe (build link)
• Windows 64-bit: lmms-1.2.3-750%2Bgb1a9050a3-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-1.2.3-750%2Bgb1a9050a3-mac10.13.dmg (build link)

🤖

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