Add SSH-tunneling to the desktop client as well

[bk138]

• [ ] include libsshtunnel into build
• [ ] create extended connection setup dialog

[bk138]

Task 1: include libsshtunnel into build

So the situation is as follows

	Android	Linux	Mac	Win	Notes
zlib	in NDK	from flatpak	in MacOS?	-
libjpeg-turbo	add_subdir	from flatpak	brew.sh	-	❗ newev versions don't build as subdir
libressl	add_subdir	openssl from flatpak	brew.sh	-	requires extra prepare step
libssh2	add_subdir	-	-	-
libsshtunnel	add_subdir	-	-	-
libvncclient	add_subdir	add_subdir	add_subdir	add_subdir
wxwidgets	N/A	via flatpak-build	brew.sh	downloaded binary
libwxservdisc	N/A	add_subdir	add_subdir	add_subdir

Ideally, we'd have a single way of managing deps for all platforms, esp. when taking into account #250. Approaches:

• ⛔ CMake FetchContent: NO, uses add_subdirectory internally, no-go for libjpeg-turbo
• ⛔ Native package managers: different per platform, not for Android
• stage 2 contenders:
  • ⛔ Hunter ? -> throws errors, could not get it to work quickly
  • Conan: rather not, too much Python. For Android ?
  • CMake ExternalProject ?

After step 1 - Android fixup

	Android	Linux	Mac	Win	Notes
zlib	in NDK	from flatpak	in MacOS?	-
libjpeg-turbo	externalproject	from flatpak	brew.sh	-	can upgrade now
libressl	externalproject	openssl from flatpak	brew.sh	-	extra prepare step in cmake now
libssh2	add_subdir	-	-	-
libsshtunnel	add_subdir	-	-	-
libvncclient	add_subdir	add_subdir	add_subdir	add_subdir
wxwidgets	N/A	via flatpak-build	brew.sh	downloaded binary
libwxservdisc	N/A	add_subdir	add_subdir	add_subdir