Usability improvements and bugfixes

Fixes for the following issues:
Make the "paste unstructured text" action available via intention
Use different character for end-of-line like symbols
Cannot surround an nlist (e.g. with horz or vert coll)
Split intention doesn't work on a single word
Add an intention to convert between vertical collection and horizontal collection
Change appearance of a line
Undo functionality missing in antiquotations
Add intentions for convenience
Add plugin description that is visible in MPS
Animated gifs for feature showcase

Migrate to MPS 2018.2

nightly-0023

Update build  script to MPS 2018.2

Addition of plaintextflow functionality

A first version of the plaintextflow language.
This language introduces concepts for fine-grained control over whitespace.
The concepts are packaged as a separate language and do not affect users of plaintextgen.
Thanks to @klemensschindler for the new functionality!

Fix empty zip file creation during release

Use zip -r to get the files in the subdirectory
Make directory-structure only one deep to comply with MPS Extensions and other MPS plugins

Make build artifact in same format as MPS Extensions / mbeddr.platform

Thanks to @bartvdrmeulen

Migrate to MPS 2018.1

thanks to Václav Pech from JetBrains for the migration!

Migrate to MPS 2017.3

thanks to Václav Pech from JetBrains for the migration!

Migrate to MPS 2017.2

nightly-0017

Fix file name in build script

Switch to MPS 2017+ rolling versioning number scheme

nightly-0016

And another fix for the build script

Reflect name of compatible MPS version in plugin zip filename for MPS 3.4

nightly-0015

Reflect MPS compatibility in zip of plugin