MDL-43074 assignfeedback_editpdf: Use escapeshellarg for all args in exec command.

This is to fix a reported problem with windows compatibility (e.g. when ghostscript is installed in
a folder with spaces in the name).
MDL-42625 behat: Make behat pacient

* When looking for texts inside the page or inside
  other containers we should wait until the elements
  are visible.
* Same when expanding tree nodes.
* Normalizing loops to spin() function using
  behat_base::TIMEOUT and behat_base::EXTENDED_TIMEOUT,
  leaving TIMEOUT for DOM load processes and
  EXTENDED_TIMEOUT for long processes that involves JS
  too.
* Add page load waits between actions that involves
  reloading the page.
MDL-43025 EditPDF: Increase memory limit when reading source pdfs.
Merge branch 'w50_MDL-43137_m26_openbasedir' of https://github.com/skodak/moodle into MOODLE_26_STABLE
Merge branch 'MDL-43092_M26' of git://github.com/lazydaisy/moodle into MOODLE_26_STABLE
Merge branch 'w50_MDL-42973_m26_addonupd' of https://github.com/skodak/moodle into MOODLE_26_STABLE
Merge branch 'wip-mdl-43224-m26' of https://github.com/rajeshtaneja/moodle into MOODLE_26_STABLE
Merge branch 'wip-MDL-43175-26' of git://github.com/abgreeve/moodle into MOODLE_26_STABLE
MDL-40003 mod_assign: Fixed submission comments not being restored
MDL-43252 Group menu in wrong order when groupings used
Merge branch 'MDL-40003-26' of git://github.com/FMCorz/moodle into MOODLE_26_STABLE
Merge branch 'MDL-43152-26' of git://github.com/andrewnicols/moodle into MOODLE_26_STABLE
Merge branch 'MDL-43194-26' of git://github.com/andrewnicols/moodle into MOODLE_26_STABLE
Merge branch 'm26_MDL-43159_Wrong_Naming_Convention_For_Variable' of https://github.com/scara/moodle into MOODLE_26_STABLE
Merge branch 'MDL-43164-26' of git://github.com/andrewnicols/moodle into MOODLE_26_STABLE
MDL-43279 block_blog_recent: fixed undefined property notice
MDL-25473 mediaplayer - prevent embedded videos from looping
Merge branch 'mdl42487-moodle26' of https://github.com/tlock/moodle into MOODLE_26_STABLE
Merge branch 'MDL-43131_26' of git://github.com/timhunt/moodle into MOODLE_26_STABLE
Merge branch 'MDL-43215_m26' of https://github.com/markn86/moodle into MOODLE_26_STABLE