Use unique heatmap name, and other feature updates

[ycl6]

A PR with similar changes has been submitted before to sqjin/CellChat#696 to fix an issue with combining heatmaps that are produced by netVisual_heatmap.

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This PR (1) resolve the issue with duplicated Heatmap object name when joining 2 or more Heatmap objects created by the netVisual_heatmap function, by create a unique ht.name name and passed to the Heatmap function.

When creating a heatmap list in the later version of ComplexHeatmap, each Heatmap object should have a unique name, if not, a warning message such as Heatmap/annotation names are duplicated is produced (as noted in sqjin/CellChat#170).

This PR (2) adds the ability for users to assign legend title to each heatmap using legend.title. This is useful when creating plots showing heatmap side-by-side to compare number/strength of interactions from multiple datasets, with legend named specifically for each dataset.

This PR (3) re-introduce the paramaters for users to change the width and height of the heatmap body.

Minor changes are to standardise the font size of the legend title and label from fontsize = 8 to use font.size which default to 8.