Grants access to:
* context match condition for context snippets (via snip.visual_text and
snip.visual_mode);
* pre/post actions (via same variable);
* context match condition to (!) lastly selected placeholder, so it is
possible now to use autotrigger snippets, that are activated by
simply typing letter while tabstop is selected;
* python interpolations to lastly selected placeholder;
Due to performance degradation reported in #552.
This reverts commit 490430f91d600807272c99e9fa1f495e33d1df43, reversing
changes made to cf2c473a8ea003cb713b3b84138c1689f4a09be8.
