Removed usage of withStyles in Dialog component

[rubenthoms]

Removed usage of withStyles in Dialog component in order to avoid conflicts with webviz-subsurface-components.

Both packages use MaterialUI v4 which again uses jss under the hood with its makeStyles / withStyles functions. jss automatically adds style nodes to the DOM containing CSS with auto-generated class names (e.g. jss1, jss2 etc.). However, since both packages did make use of the makeStyles function and both jss instances did not know of one another (and jss does not check for duplicates in the DOM when generating class names), class naming conflicts occured (e.g. the first dynamically created class in wcc would have the same name as the first in wsc - jss1). This lead to the jss1 class in wcc being applied to all elements in wsc with the same class name (and vice versa) and, hence, preventing pointer-events (since the only class added with jss in wcc was one preventing the background of the dialog accepting pointer-events in order to make the plugin behind the dialog remain clickable/usable when no backdrop was set).

This issue is temporarily fixed by removing the usage of withStyles in the Dialog component. However, the problem itself remains and might lead to further bugs that are difficult to debug. A general solution to this issue might be to upgrade to MaterialUI v5 which no longer uses jss but emotion. However, it is unknown if emotion has an implementation for checking for duplicates and works better than jss.

[jorgenherje]

Looks good to me! 👍

[rubenthoms]

Closes #236.