#171 special recently_modified for dataframe content

[Michele Gubian]

hi James,
this is an old issue, but recently I stumbled into small GUI bugs that then I realise won't fix because of this general lazyNut issue that I want to put black on white below.

The GUI needs a new recently_modified command to query dataframe content, namely:
recently_modified should tell if the description of a dataframe has changed, while:
data_recently_modified or something like that should tell if the content of a dataframe has changed.
This will propagate to plots thanks to the already existing dependencies mechanism. In this way the GUI can minimise plot gets. One of the many examples is when you change a hint in the description of a df. That triggers a recently_modified and the GUI assumes the worst case, viz that the content of df, and as a consequence of all the plots depending on it, has been modified. At the moment the GUI code is littered with obscure, brittle and often wrong workarounds to avoid sending too many gets. But it has reached a limit. So I am not going to push further until lazyNut will expose something like data_recently_modified.
Thanks..