Initialize window content in constructed handler

The content of the main window is initialized in the init handler, and
this is not ideal. The init handler is called early in the construction
process and for example properties have not been set yet.

We want to be able to create a window with a dynamic number of rows,
columns and flowers; which should most likely be configured via
properties, so we have to postpone the initialization of the window
contents to a later stage. This is what the constructed handler can be
used for, since it runs last. Implement a handler for constructed and
move the initialization there.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>