Raw content return for Grid DataColumn & Column

Need to have a method  `getDataCellContent` to return raw parsed data cell content. This is useful when extending these Columns, for processing any other data before formatting via `renderDataCellContent`.
Raw content return for Grid DataColumn & Column

Need to have a method  `getDataCellContent` to return raw parsed data cell content. This is useful when extending these Columns, for processing any other data before formatting via `renderDataCellContent`.
Raw content return for Grid DataColumn & Column
Raw content return for Grid DataColumn & Column
Removed unnecessary blank space
Removed unnecessary blank space
first draft of the extension generator
WIP merge ActiveRelation into ActiveQuery

allow extending only one class to add scopes, fixes #2146

TODO:

- [ ] adjust guide docs
- [ ] adjust README files of extensions
- [ ] finish work and fix test breaks
made createQuery() config param optional
Add ability to downgrade migrations by the absolute apply time
Add changelog line related to #2499
updated timestamp behavior in advanced app to renaming
fixed phpdoc errors and updated properties
updated classmap
added missing use statement
Merge pull request #2500 from lucianobaraglia/master

Some minimal doc blocks changes for consitency
doc update about behavior.
Merge pull request #2499 from resurtm/migrate-to-datetime

Add an ability to downgrade migrations by the absolute apply time
Fixes #2487
Fixes #2460: custom listview/gridview summary was broken