pgAdmin 4 commit: Allow to load the stylesheets of addons at runtime.

Allow to load the stylesheets of addons at runtime.

- Introduced a PgAdminModule method 'get_addon_stylesheets' to get the
  list of addons exposed by any addons for a particular module.
- Renamed the 'get_javascript_entries' method to 'get_addon_javascripts'
  to match the name with 'get_addon_stylesheets'.

Branch
------
WEBPACK_DEVEL

Details
-------
https://git.postgresql.org/gitweb?p=pgadmin4.git;a=commitdiff;h=db98b85b66e12189dfecf2ad3990425dfe7a92b7

Modified Files
--------------
web/pgadmin/__init__.py                          | 34 +++++++++++++++++++++---
web/pgadmin/browser/__init__.py                  |  3 ++-
web/pgadmin/browser/templates/browser/index.html |  7 +++++
web/pgadmin/tools/__init__.py                    |  4 +--
web/pgadmin/utils/__init__.py                    | 31 ++++++++++++++++++---
5 files changed, 68 insertions(+), 11 deletions(-)