Make flycheck-pos-tip-hide-messages work without minor mode

[bcc32]

This way, the package works as expected even when the user enables it
by customizing flycheck-display-errors-function (or happens to save
such a customization permanently after enabling
flycheck-pos-tip-mode).

Fix #36.

The new implementation strategy is to add
flycheck-pos-tip-hide-messages to the appropriate hooks temporarily
when the display function is called, and remove it when the messages
are actually dismissed. This avoids depending on the minor mode being
enabled at all.

Now, the only job of the minor mode is to be able to interactively
toggle the value of flycheck-display-errors-function.

[cpitclaudel]

Shouldn't this be out of the unless block?

[bcc32]

No, that doesn't quite work. flycheck-pos-tip-hide-messages is called in post-command-hook so invoking any command that doesn't happen to move point would cause the hook to be removed if it was outside of the unless block.

Consider the following example operations:

1. Move point onto a highlighted error. Wait a bit, then the tooltip will be displayed.
2. Press C-h . (or even just C-g, etc.) which is a command that does not change current-buffer, buffer-modified-tick, or point
3. Move point off of the error. This should hide the tooltip

With the suggested snippet moved outside of the unless block, (3) does not hide the error as it should.

(Sorry for the delay, I had a lot of personal stuff going on recently.)

[cpitclaudel]

Woops, I missed this PR. It looks good to me, I left one comment