Add panels plotMediation, plotPrevalence and plotTile

[RiboRings]

Recently, getMediation and plotMediation were implemented in mia/miaViz, so it would be nice to create corresponding panels for mediation analyses in iSEEtree.

[RiboRings]

PrevalencePlot was added, but it can still be improved in terms of:

• aesthetics control
• flexibility of shown prevalence/detection lines
• other two layouts should be added (miaViz plotRowPrevalence and plotPrevalentAbundance functions)