Remove dependencies to external binarys

[ygerlach]

I have started to remove some dependencies to external binarys, as easy alternatives are available.

This makes those actions of timeshift faster and more efficient, as fewer syscalls are required to archive the same result. It also allows timeshift to have more detailed knowledge about the errors that might happen.

i have lots to add in future pull requests:

• remove more dependencies to simple programms, such as cp, ln, touch, mount, umount,...
• move to a more event driven application design allowing to use async functions of glib
• fix compiler warnings
• try to make Main.vala smaller, by moving some functionality into new or other classes
• make stuff like mountpoints and backup folder in external drives configureable
• move the application look the shared memory, to be automatically cleared on reboot and/or just use file locks
• get rid of crontab in favor of systemd timers
• there are some bigger features i have in mind, but i will create a issue for those in time, to allow for some discussion