Supporting more boot setups

[DonyorM]

Bootloaders are complex things. Right now WereSync only support certain setups of grub. More bootloading setups would be much appreciated.

[DonyorM]

The feat-bootloader branch contains many of the updates in this issue. Grub is supported on single-boot setups, and mostly works on dual-boot ones, but has some issue.

I would still like to support more varied bootloaders.