[schema migration] import refactoring to fix #1109558 and enhances things on the way the main pb demonstrated by #1109558 was due to the fact that in-memory schema was updated in commit_event of operations. This is undesired in most cases, since we want the modification to be taken into account in the interval between the modification detection and the commit_event. I've fixed this by merging in-memory schema / database alteration operations for most important changes, doing in-memory schema changes as they are detected and implementing a revertcommit_event method to revert them if necessary (with exception for removal of stuff from the schema, where this is simply done in a postcommit_event methods). Also, I've benefited from this to support reverting of database alteration for some operations (more to be done there), and to move so system source alteration code to the native source code for a nicer design. There may be some more stuff in syncschema.py that would benefit from similar changes, but most important things are done (at least to close #1109558, w/ unittest_syncschema and unittest_migration green).

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# Put here information about external resources / styles used by your cube,
# or to overides existing UI properties.
#
# Existing properties are available through the `sheet` dictionary available
# in the global namespace. You also have access to a `data` function which
# will return proper url for resources in the 'data' directory.
#
# /!\ this file should not be imported /!\
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# CSS stylesheets to include in HTML headers
# uncomment the line below to use template specific stylesheet
# STYLESHEETS = sheet['STYLESHEETS'] + [data('cubes.%(cubename)s.css')]