Document usage of multiple platforms (issue #85)

See also issue #85 on GitHub:
  #85

Issue #128: Make saving of GUI font optional

Issue 128 on GitHub:
  #128

Merge pull request #81: Add g:session_autosave_to (override) option

See pull request 81 on GitHub:
  #81

Enable silencing automatic periodic sessions saving

Refer to issue 120 on GitHub for details:
  #120

Rephrase prompts for automatic session save/restore

I hardly ever see the prompts emitted by the vim-session plug-in because
I've configured everything to work the way I want it to automatically,
but while updating the screenshots of the prompts in response to issue
#117 I noticed that the phrasing of the prompts could be improved, hence
this change.

See also issue 117 on GitHub:
  #117

Merge pull request #104: Make GUI dialogs easier to read

Note there is one change in the merge commit which isn't in the original
pull request: I've removed the Unicode smart quotes, because I've learned
that unconditional Unicode character usage in my Vim plug-ins is a big
no-no waiting for unhappy users to report they see strange characters
in the Vim plug-in's output :-). Apart from that I fully agree with
the changes implemented by @roryokane.

Move "session is empty?" check to separate function

Merge pull request 116: Fix --servername option when using vim-startify

See also pull request 116 on GitHub:
  #116

Which came forth from issue 115 on GitHub:
  #115

Add option to disable session locking (issue #112)

Please refer to issue #112 on GitHub:
  #112

Automatically store lock files in /var/lock when possible (issue #110)

This is basically a follow up to issue #101 with the idea that sane
defaults should be provided when possible - most users don't read the
documentation and even when they do they might still not find what
they are looking for (and I don't really blame them, I'm exactly the
same :-).

Issue #101 on GitHub: #101
Issue #110 on GitHub: #110